Liquid crystal (LC) devices are presently utilized to produce numerous active structures, including color shutters, tunable polarization interference filters, light valves, and complex amplitude modulators. However, the chromaticity of liquid crystal retarders places limitations on the spectral band over which they function properly. Two factors contribute to the wavelength sensitivity, or chromaticity, of a waveplate: (1) dispersion, which is the wavelength dependence of the dielectric anisotropy, and (2) the explicit inverse wavelength dependence of retardation. Both components serve to increase the retardation at shorter wavelengths. A birefringent material with a particular retardation at the design wavelength will have greater retardation at shorter wavelengths and less retardation at longer wavelengths.
To examine the effect of retarder chromaticity, consider the polarization switches known in the art using rotative liquid crystal elements. In the conventional approach to shuttering light with a planar aligned chiral smectic liquid crystal (CSLC), the molecular director of an LC half-wave plate switches between 0 and .pi./4 orientations with respect to bounding crossed polarizers, as shown in FIG. 1. The half-wave retardance center wavelength is selected to provide maximum transmission at the operating wavelength. In display and cameras, it is selected to optimally span the visible spectrum. However, the on-state bandwidth is narrow, and invariably has poor red/blue transmission. Furthermore, small spatial thickness variations of the CSLC film produce highly visible color variations. FIG. 2 shows the computer model on-state transmission of a prior art CSLC shutter utilizing a 500 nm half-wave plate. The model includes the effect of birefringence dispersion. Note the extreme chromaticity of the shutter; the transmission varies by a factor of two over the visible spectrum.
More elaborate active achromatic structures are described incorporating multiple active elements. Dahl et al. (PCT Publication No. WO 90/09614 1990!) describe an LC shutter which incorporates chromatic compensation. Here, two analog CSLC half-wave plates are positioned between a pair of crossed polarizers. The two plates are symmetrically modulated, the first with orientation .THETA. and the second with orientation 90-.THETA., to provide a more achromatic response than a single cell shutter. Because this device requires two active CSLC cells it has not achieved wide use.
Chromaticity compensation using passive multilayers of identical retarder material was addressed by S. Pancharatnam, Proc. Indian Acad. Sci. A41, 137 1955!, by A.M. Title, Appl. opt. 14, 229 1975!, and by C. J. Koester, J. Opt. Soc. Am. 49, 405, 1959!, all of which are herein incorporated by reference in their entirety.
Three-waveplate structures are described by Pancharatnam which function as achromatic retarders. These structures consist of three films of identical material, and design parameters are provided to allow construction of devices with arbitrary retardance values. It is noteworthy that a minimum of three elements are required in order to construct a Pancharatnam compound retarder.
By mechanically rotating a Pancharatnam achromatic half-wave retarder unit, wavelength insensitive reorientation of linear polarization is feasible. Electromechanical rotation of such compound half-wave retarders has been used extensively to tune polarization interference filters for astronomical imaging spectrometers. Of course, a solid-state version of this would require electrooptic rotation of three retarders synchronously.
A simplified solid-state achromatic retarder was recently invented by Sharp and Johnson (U.S. patent application Ser. No. 08/419,593). This Pancharatnam based design comprises a single rotative LC half-wave retarder bounded by passive half-wave retarders. The inventors realized that, for a few specific orientations, rotation of a single element is sufficient to effectively rotate the optic axis of the entire structure.
Half-wave retarders, including the Pancharatnam three-element half-wave retarder unit, convert an incident plane polarized beam of arbitrary orientation .THETA. with respect to the retarder axis to a beam of orientation -.THETA.. Koester realized that an "achromatic rotator" can be formed using two linear half-wave retarders. The achromatic rotator requires input polarization at a fixed orientation and provides rotation of the plane of polarization through a fixed angle. Unlike an achromatic retarder it cannot accept light polarized with arbitrary orientation. Therefore, mechanical or solid state rotation of the waveplates would destroy the function of the achromatic rotator, and active switching of the rotator has not been described.